The military judge hearing the court-martial of US soldier Bradley Manning has refused to dismiss the most severe charge against him, aiding the enemy.

That is just one of the 21 counts Manning faces as he is accused of the biggest leak of classified material in US history.

"He was knowingly providing intelligence to the enemy," said judge Colonel Denise Lind in rejecting Manning's lawyer's motion to dismiss that charge.

Manning, 25, is charged with sharing more than 700,000 classified files, combat videos and State Department cables with the anti-secrecy website WikiLeaks while serving as a low-level intelligence analyst in Iraq in 2009 and 2010.

Defence lawyer David Coombs had argued Manning was guilty of negligence but not the "general evil intent" standard required to justify the heavy charge which carries a possible life sentence.

Over the course of the trial, defence lawyers have sought to show that the slightly built Manning was naive but well-intentioned in seeking to inform Americans about the reality of the wars in Afghanistan and Iraq.

The trial will also hear from prosecution witnesses in a rebuttal phase following the close of the defence phase last week.